OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..800
F. Ruskey, Necklaces, Lyndon words, De Bruijn sequences, etc. [Cached copy, with permission, pdf format only]
Eric Weisstein's World of Mathematics, Necklace
Wikipedia, Necklace (combinatorics)
FORMULA
a(n) = 1/(7*n) * Sum_{d|n} C(7*n/d,n/d) * A000010(d) for n>0, a(0) = 1.
a(n) ~ 7^(7*n-1/2) / (2 * sqrt(3*Pi) * 6^(6*n) * n^(3/2)). - Vaclav Kotesovec, Aug 22 2015
MAPLE
with(numtheory):
a:= n-> `if`(n=0, 1, add(binomial(7*n/d, n/d)
*phi(d), d=divisors(n))/(7*n)):
seq(a(n), n=0..25);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Aug 21 2015
STATUS
approved